1. Field of the Invention
This invention relates generally to helium compressor units for use in cryogenic refrigeration systems operating on the Gifford McMahon (GM) and Brayton cycles. More particularly, the invention relates to dual after-coolers that provide redundancy between water cooling and air cooling if there is a blockage in the water or air supply.
2. Description of the Related Art
The basic principal of operation of a GM cycle refrigerator is described in U.S. Pat. No. 2,906,101 to McMahon, et al. A GM cycle refrigerator consists of a compressor that supplies gas at a discharge pressure to an inlet valve which admits gas to an expansion space through a regenerator, expands the gas adiabatically within a cold end heat exchanger where it receives heat from an object being cooled, then returns the gas at low pressure to the compressor through the regenerator and an outlet valve. The GM cycle has become the dominant means of producing cryogenic temperatures in small commercial refrigerators primarily because it can utilize mass produced oil-lubricated air-conditioning compressors to build reliable, long life, refrigerators at minimal cost. GM cycle refrigerators operate well at pressures and power inputs within the design limits of air-conditioning compressors, even though helium is substituted for the design refrigerants. Typically, GM refrigerators operate at a high pressure of about 2 MPa, and a low pressure of about 0.8 MPa. The cold expander in a GM refrigerator is typically separated from the compressor by 5 m to 20 m long gas lines. The expanders and compressors are usually mounted indoors and the compressor is usually cooled by water, most frequently water that is circulated by a water chiller unit. Air cooled compressors that are mounted indoors are typically cooled by air conditioned air which is in the temperature range of 15° C. to 30° C.
A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a heat exchanger, from which gas is admitted to an expansion space through an inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns it to the compressor at a low pressure through the heat exchanger. Brayton cycle refrigerators operating at cryogenic temperatures can also be designed to operate with the same compressors that are used for GM cycle refrigerators.
Disadvantageously, compressors designed for air-conditioning service require additional cooling when compressing helium because monatomic gases including helium get a lot hotter when compressed than standard refrigerants. U.S. Pat. No. 7,674,099 describes a means of adapting a scroll compressor manufactured by Copeland Corp. to injecting oil along with helium into the scroll such that about 2% of the displacement is used to pump oil. Approximately 70% of the heat of compression leaves the compressor in the hot oil and the balance in the hot helium.
The Copeland compressor is oriented horizontally and requires an external bulk oil separator to remove most of the oil from the helium. Another scroll compressor that is widely used for compressing helium is manufactured by Hitachi Inc. The Hitachi compressor is oriented vertically and brings the helium and oil directly into the scroll through separate ports at the top of the compressor and discharges it inside the shell of the compressor. Most of the oil separates from the helium inside the shell and flows out of the shell near the bottom while the helium flows out near the top. Helium compressor systems that use the Copeland and Hitachi scroll compressors have separate channels in one or more after-coolers for the helium and oil. Heat is transferred from the oil and helium to either air or water. The cooled oil is returned to the compressor and the cooled helium passes through a second oil separator and an adsorber before flowing to the expander. U.S. Pat. No. 7,674,099 shows after-cooler 8 as being a single heat exchanger cooled by water. This is a typical arrangement for helium compressor systems that operate indoors where chilled water is available. Some helium compressor systems have air cooled after-coolers located indoors but they put an extra heat load on the air conditioning system so it is more typical to have air cooled after-coolers mounted outdoors, either integral with the compressor or separate from the compressor. U.S. Pat. No. 8,978,400 shows an arrangement with a Hitachi scroll compressor that has the oil cooler outdoors cooled by air and all the other components indoors with the helium cooled either by air or water. As explained in the '400 patent, keeping all of the components that have helium in them indoors in an air condition environment, where the temperature is in the range of 15° C. to 30° C., minimizes the contaminants that evolve from hot oil and increases the life of the final adsorber.
Patent DE3023925 describes a helium compressor system with a water cooled after-cooler which has an option to cool the water with an air cooled heat exchanger and a pump that circulates the water. This arrangement adds the temperature difference of the helium/oil-to-water heat exchanger to the water-to-air heat exchanger and results in higher helium and oil temperatures that release more contaminants into the helium.